Classics in the History of Psychology

The Reflex Arc Concept in Psychology
John Dewey (1896)

First published in Psychological Review, 3,
357-370.

That the greater demand for a unifying principle and controlling
working hypothesis in psychology should come at just the time
when all generalizations and classifications are most questioned
and questionable is natural enough. It is the very cumulation
of discrete facts creating the demand for unification that also
breaks down previous lines of classification. The material is
too great in mass and too varied in style to fit into existing
pigeon-holes, and the cabinets of science break of their own dead
weight. The idea of the reflex arc has upon the whole come nearer
to meeting this demand for a general working hypothesis than any
other single concept. It being admitted that the sensori-motor
apparatus represents both the unit of nerve structure and the
type of nerve function, the image of this relationship passed
over into psychology, and became an organizing principle to hold
together the multiplicity of fact.

In criticising this conception it is not intended to make a plea
for the principles of explanation and classification which the
reflex arc idea has replaced; but, on the contrary, to urge that
they are not sufficiently displaced, and that in the idea of the
sensori-motor circuit, conceptions of the nature of sensation
and of action derived from the nominally displaced psychology
are still in control.

The older dualism between sensation and idea is repeated in the
current dualism of peripheral and central structures and functions;
the older dualism of body and soul finds a distinct echo in the
current dualism of stimulus and response. Instead of interpreting
the character of sensation, idea and action from their place and
function in the sensory-motor circuit, we still incline to interpret
the latter from our preconceived and preformulated ideas of rigid
distinctions between sensations, thoughts and acts. The sensory
stimulus is one thing, the central activity, standing for the
idea, and the motor discharge, standing for the act proper, is
a third. As a result, the reflex arc is not a comprehensive, or
organic unity, but a patchwork of disjointed parts, a mechanical
conjunction of unallied processes. What is needed is that the
principle underlying the idea of the reflex arc as the fundamental
psychical unity shall react into and determine the values of its
constitutive factors. More specifically, what is wanted is that
sensory stimulus, central connections and motor responses shall
be viewed, not as separate and complete entities in themselves,
but as divisions of labor, function factors, within the single
concrete whole, now designated the reflex arc.

What is the reality so designated? What shall we term that which
is not sensation-followed-by-idea-followed-by-movement,. but which
is primary; which is, as it were, the psychical organism of which
sensation, idea and movement are the chief organs? Stated on the
physiological side, this reality may most conveniently be termed
coördination. This is the essence of the facts held together
by and subsumed under the reflex arc concept. Let us take. for
our example, the familiar child-candle instance. (James, Psychology,
Vol. I, p. 25.) The ordinary interpretation would say the sensation
of light is a stimulus to the grasping as a response, the burn
resulting is a stimulus to withdrawing the hand as response and
so on. There is, of course, no doubt that is a rough practical
way of representing the process. But when we ask for its psychological
adequacy, the case in quite different. Upon analysis, we find
that we begin not with a sensory stimulus. but with a sensori-motor
coördination, the optical-ocular, and that in a certain sense
it is the movement which is primary, and the sensation which is
secondary, the movement of body, head and eye muscles determining
the quality of what is experienced. In other words, the real beginning
is with the act of seeing; it is looking, and not a sensation
of light. The sensory quale gives the value of the act, just as
the movement furnishes its mechanism and control, but both sensation
and movement lie inside, not outside the act.

Now if this act, the seeing stimulates another act, the reaching,
it is because both of these acts fall within a larger coördination;
because seeing and grasping have been so often bound together
to reinforce each other, to help each other out, that each may
be considered practically a subordinate member of a bigger coördination.
More specifically, the ability of the hand to do its work will
depend, either directly or indirectly, upon its control, as well
as its stimulation, by the act of vision. If the sight did not
inhibit as well as excite the reaching, the latter would be purely
indeterminate, it would be for anything or nothing, not for the
particular object seen. The reaching, i n turn, must both stimulate
and control the seeing. The eye must be kept upon the candle if
the arm is to do its work; let it wander and the arm takes up
another task. In other words, we now have an enlarged and transformed
coördination; the act is seeing no less than before, but
it is now seeing-for-reaching purposes. There is still a sensori-motor
circuit, one with more content or value, not a substitution of
a motor response for a sensory stimulus.[1]

Now take the affairs at its next stage, that in which the child
gets burned. It is hardly necessary to point out again that this
is also a sensori-motor coördination and not a mere sensation.
It is worth while, however, to note especially the fact that it
is simply the completion, or fulfillment, of the previous eye-arm-hand
coördination and not an entirely new occurrence. Only because
the heat-pain quale enters into the same circuit of experience
with the optical-ocular and muscular quales, does the child learn
from the experience and get the ability to avoid the experience
in the future.

More technically stated, the so-called response is not merely
to the stimulus; it is into it. The burn is the
original seeing, the original optical-ocular experience enlarged
and transformed in its value. It is no longer mere seeing; it
is seeing-of-a-light-that-means-pain-when-contact-occurs. The
ordinary reflex arc theory proceeds upon the more or less tacit
assumption that the outcome of the response is a totally new experience;
that it is, say, the substitution of a burn sensation for a light
sensation through the intervention of motion. The fact is that
the sole meaning of the intervening movement is to maintain, reinforce
or transform (as the case may be) the original quale that we do
not have the replacing of one sort of experience by another, but
the development (or as it seems convenient to term it) the mediation
of an experience. The seeing, in a word, remains to control the
reaching, and is, in turn, interpreted by the burning.[2]

The discussion up to this point may be summarized by saying that
the reflex arc idea, as commonly employed, is defective in that
it assumes sensory stimulus and motor response as distinct psychical
existences, while in reality they are always inside a coördination
and have their significance purely from the part played in maintaining
or reconstituting the coördination; and (secondly) in assuming
that the quale of experience which precedes the 'motor' phase
and that which succeeds it are two different states, instead of
the last being always the first reconstituted, the motor phase
coming in only for the sake of such mediation. The result is that
the reflex arc idea leaves us with a disjointed psychology, whether
viewed from the standpoint of development in the individual or
in the race, or from that of the analysis of the mature consciousness.
As to the former, in its failure to see that the arc of which
it talks is virtually a circuit, a continual reconstitution, it
breaks continuity and leaves us nothing but a series of jerks,
the origin of each jerk to be sought outside the process of experience
itself, in either an external pressure of 'environment,' or else
in an unaccountable spontaneous variation from within the 'soul'
or the 'organism.'[3]As to the latter,
failing to see unity of activity, no matter how much it may prate
of unity, it still leaves us with sensation or peripheral stimulus;
idea, or central process (the equivalent of attention); and motor
response, or act, as three disconnected existences, having to
be somehow adjusted to each other, whether through the intervention
of an extraexperimental soul, or by mechanical push and pull.

Before proceeding to a consideration of the general meaning for
psychology of the summary, it may be well to give another descriptive
analysis, as the value of the statement depends entirely upon
the universality of its range of application. For such an instance
we may conveniently take Baldwin's analysis of the reactive consciousness.
In this there are, he says (Feeling and Will, p. 60), "three
elements corresponding to the three elements of the nervous arc.
First, the receiving consciousness, the stimulus -- say a loud,
unexpected sound; second, the attention involuntarily drawn, the
registering element; and, third, the muscular reaction following
upon the sound -- say flight from fancied danger." Now, in
the first place, such an analysis is incomplete; it ignores the
status prior to hearing the sound. Of course, if this status is
irrelevant to what happens afterwards, such ignoring is quite
legitimate. But is it irrelevant either to the quantity or the
quality of the stimulus?

If one is reading a book, if one is hunting, if one is watching
in a dark place on a lonely night, if one is performing a chemical
experiment, in each case, the noise has a very different psychical
value; it is a different experience. In any case, what proceeds
the 'stimulus' is a whole act, a sensori-motor coördination.
What is more to the point, the 'stimulus' emerges out of this
coördination; it is born from it as its matrix; it represents
as it were an escape from it. I might here fall back upon authority,
and refer to the widely accepted sensation continuum theory, according
to which the sound cannot be absolutely ex abrupto from
the outside, but is simply a shifting of focus of emphasis, a
redistribution of tensions within the former act; and declare
that unless the sound activity had been present to some extent
in the prior coördination, it would be impossible for it
now to come to prominence in consciousness. And such a reference
would be only an amplification of what has already been said concerning
the way in which the prior activity influences the value of the
sound sensation. Or, we might point to cases of hypnotism, mono-ideaism
and absent-mindedness, like that of Archimedes, as evidences that
if the previous coördination is such as rigidly to lock the
door, the ' auditory disturbance will knock in vain for admission
to consciousness. Or, to speak more truly in the metaphor, the
auditory activity must already have one foot over the threshold,
if it is ever to gain admittance.

But it will be more satisfactory, probably, to refer to the biological
side of the case, and point out that as the ear activity has been
evolved on account of the advantage gained by the whole organism,
it must stand in the strictest histological and physiological
connection with the eye, or hand, or leg, or what-.; ever other
organ has been the overt center of action. It is absolutely impossible
to think of the eye center as monopolizing' consciousness and
the ear apparatus as wholly quiescent, What happens is a certain
relative prominence and subsidence as between the various organs
which maintain the organic equilibrium.

Furthermore, the sound is not a mere stimulus, or mere. sensation;
it again is an act, that of hearing. The muscular response is
involved in this as well as sensory stimulus; that is, there is
a certain definite set of the motor apparatus involved in hearing
just as much as there is in subsequent running away. The movement
and posture of the head, the tension of the ear muscles, are required
for the ' reception' of the sound. It is just as true to say that
the sensation of sound arises from a motor response as that the
running away is a response to the sound. This may be brought out
by reference to the fact that Professor Baldwin, in the passage
quoted, has inverted the real order as between his first and second
elements. We do not have first a sound and then activity of attention,
unless sound is taken as mere nervous shock or physical event,
not as conscious value. The conscious sensation of sound depends
upon the motor response having already taken place; or, in terms
of the previous statement (if stimulus is used as a conscious
fact, and not as a mere physical event) it is the motor response
or attention which constitutes that, which finally becomes the
stimulus to another act. Ones more, the final 'element,' the running
away, is not merely motor, but is sensori-motor, having its sensory
value and its muscular mechanism. It is also a coördination.
And, finally, this sensori-motor coördination is not a new
act, supervening upon what preceded. Just as the 'response' is
necessary to constitute the stimulus, to determine it as sound
and as this kind of sound, of wild beast or robber, so the sound
experience must persist as a value in the running, to keep it
up, to control it. The motor reaction involved in the running
is, once more, into, not merely to, the sound. It occurs to change
the sound, to get rid of it. The resulting quale, whatever it
may be, has its meaning wholly determined by reference to the
hearing of the sound. It is that experience mediated.[4]
What we have is a circuit, not an arc or broken segment of a circle.
This circuit is more truly termed organic than reflex, because
the motor response determines the stimulus, just as truly as sensory
stimulus determines movement. Indeed, the movement is only for
the sake of determining the stimulus, of fixing what kind of a
stimulus it is, of interpreting it.

I hope it will not appear that I am introducing needless refinements
and distinctions into what, it may be urged, is after all an undoubted
fact, that movement as response follows sensation as stimulus.
It is not a question of making the account of the process more
complicated, though it is always wise to beware of that false
simplicity which is reached by leaving out of account a large
part of the problem. It is a question of finding out what stimulus
or sensation, what movement and response mean; a question of seeing
that they mean distinctions of flexible function only, not of
fixed existence; that one and the same occurrence plays either
or both parts, according to the shift of interest; and that because
of this functional distinction and relationship, the supposed
problem of the adjustment of one to the other, whether by superior
force in the stimulus or an agency ad hoc in the center
or the soul, is a purely self-created problem.

We may see the disjointed character of the present theory, by
calling to mind that it is impossible to apply the phrase 'sensori-motor'
to the occurrence as a simple phrase of description; it has validity
only as a term of interpretation, only, that is, as defining various
functions exercised. In terms of description, the whole process
may be sensory or it may be motor, but it cannot be sensori-motor.
The 'stimulus,' the excitation of the nerve ending and of the
sensory nerve, the central change, are just as much, or just as
little, motion as the events taking place in the motor nerve and
the muscles. It is one uninterrupted, continuous redistribution
of mass in motion. And there is nothing in the process, from the
standpoint of description, which entitles us to call this reflex.
It is redistribution pure and simple; as much so as the burning
of a log, or the falling of a house or the movement of the wind.
In the physical process, as physical, there is nothing which can
be set off as stimulus, nothing which reacts, nothing which is
response. There is just a change in the system of tensions.

The same sort of thing is true when we describe the process purely
from the psychical side. It is now all sensation, all sensory
quale; the motion, as psychically described, is just as much sensation
as is sound or light or burn. Take the withdrawing l of the hand
from the candle flame as example. What we have is a certain visual-heat-pain-muscular-quale,
transformed into another visual-touch-muscular-quale -- the flame
now being visible only at a distance, or not at all, the touch
sensation being altered. etc. If we symbolize the original quale
by v, the temperature by h, the accompanying muscular sensation
by m, the whole experience may be stated as vhm-vhm-vhm';
m being the quale of withdrawing, m' the sense of
the status after the withdrawal. The motion is not a certain kind
of existence; it is a sort of sensory experience interpreted,
just as is candle flame, or burn from candle flame. All are on
a par.

But in spite of all this, it will be urged, there is a distinction
between stimulus and response, between sensation and motion. Precisely;
but we ought now to be in a condition to ask of what nature is
the distinction, instead of taking it for granted as a distinction
somehow lying in the existence of the facts themselves. We ought
to be able to see that the ordinary conception of the reflex arc
theory, instead of being a case of plain science, is a survival
of the metaphysical dualism, first formulated by Plato, according
to which the sensation is an ambiguous dweller on the border land
of soul and body, the idea (or central process) is purely psychical,
and the act (or movement) purely physical. Thus the reflex arc
formulation is neither physical (or physiological) nor psychological;
it is a mixed materialistic-spiritualistic assumption.

If the previous descriptive analysis has made obvious the need
of a reconsideration of the reflex arc idea, of the nest of difficulties
and assumptions in the apparently simple statement, it is now
time to undertake an explanatory analysis. The fact is that stimulus
and response are not distinctions of existence, but teleological
distinctions, that is, distinctions of function, or part played,
with reference to reaching or maintaining an end. With respect
to this teleological process, two stages should be discriminated,
as their confusion is one cause of the confusion attending the
whole matter. In one case, the relation represents an organization
of means with reference to a comprehensive end. It represents
an accomplished adaptation. Such is the case in all well developed
instincts, as when we say that the contact of eggs is a stimulus
to the hen to set; or the sight of corn a stimulus to pick; such
also is the case with all thoroughly formed habits, as when the
contact with the floor stimulates walking. In these instances
there is no question of consciousness of stimulus as stimulus,
of response as response.

There is simply a continuously ordered sequence of acts, all adapted
in themselves and in the order of their sequence, to reach a certain
objective end, the reproduction of the species, the preservation
of life, locomotion to a certain place. The end has got thoroughly
organized into the means. In calling one stimulus, another response
we mean nothing more than that such an orderly sequence of acts
is taking place. The same sort of statement might be made equally
well with reference to the succession of changes in a plant, so
far as these are considered with reference to their adaptation
to, say, producing seed. It is equally applicable to the series
of events in the circulation of the blood, or the sequence of
acts occurring in a self-binding reaper.[5]

Regarding such cases of organization viewed as already attained,
we may say, positively, that it is only the assumed common reference
to an inclusive end which marks each member off as stimulus and
response, that apart from such reference we have only antecedent
and consequent;[6] in other words, the distinction
is one of interpretation. Negatively, it must be pointed out that
it is not legitimate to carry over, Without change, exactly. the
same order of considerations to cases where it is a questions
of conscious stimulation and response. We may, in the above
case, regard, if we please, stimulus and response each as an entire
act, having an individuality of its own, subject even here to
the qualification that individuality means not an entirely independent
whole, but a division of labor as regards maintaining or reaching
an end. But in any case, it is an act, a sensory motor coördination,
which stimulates the response, itself in turn sensori-motor, not
a sensation which stimulates a movement. Hence the illegitimacy
of identifying, as is so often done, such cases of organized instincts
or habits will the so-called reflex arc, or of transferring, without
modification, considerations valid of this serial coördination
of acts to the sensation-movement case.

The fallacy that arises when this is done is virtually the psychological
or historical fallacy. A set of considerations which hold good
only because of a completed process, is read into the content
of the process which conditions this completed result. A state
of things characterizing an outcome is regarded as a true description
of the events which led up to this outcome; when, as a matter
of fact, if this outcome had already been in existence, there
would have been no necessity for the process. Or, to make the
application to the case in hand, considerations valid of an attained
organization or coördination, the orderly sequence of minor
acts in a comprehensive coördination, are used to describe
a process, viz., the distinction of mere sensation as stimulus
and of mere movement as response, which takes place only because
such an attained organization is no longer at hand, but is in
process of constitution. Neither mere sensation, nor mere movement,
can ever be either stimulus or response; only an act can be that;
the sensation as stimulus means the lack of and search
for such an objective stimulus, or orderly placing of an act;
just as mere movement as response means the lack of and search
for the right act to complete a given coördination.

A recurrence to our example will make these formulae clearer,
As long as the seeing is an unbroken act, which is as experienced
no more mere sensation than it is mere motion (though the onlooker
or psychological observer can interpret it into sensation and
movement), it is in no sense the sensation which stimulates the
reaching; we have, as already sufficiently indicated, only the
serial steps in a coördination of acts. But now take
a child who, upon reaching for bright light (that is, exercising
the seeing-reaching coördination) has sometimes had a delightful
exercise, sometimes found something good to eat and sometimes
burned himself. Now the response is not only uncertain, but
the stimulus is equally uncertain; one is uncertain only so far
as the other is. The real problem may be equally well stated
as either to discover the right stimulus, to constitute the stimulus,
or to discover, to constitute, the response. The question of whether
to reach or to abstain from reaching is the question what sort
of a bright light have we here? Is it the one which means playing
with one's hands, eating milk, or burning one's fingers? The stimulus
must be constituted for the response to occur. Now it is at precisely
this juncture and because of it that the distinction of sensation
as stimulus and motion as response arises.

The sensation or conscious stimulus is not a thing or existence
by itself; it is that phase of a coördination requiring attention
because, by reason of the conflict within the coördination,
it is uncertain how to complete it. It is to doubt as to the next
act, whether to reach or no, which gives the motive to examining
the act. The end to follow is, in this sense, the stimulus. It
furnishes the motivation to attend to what has just taken place;
to define it more carefully. From this point of view the discovery
of the stimulus is the ' response' to possible movement as 'stimulus.'
We must have an anticipatory sensation, an image, of the movements
that may occur, together with their respective: values, before
attention will go to the seeing to break it up as a sensation
of light, and of light of this particular kind. It is the initiated
activity of reaching, which, inhibited by the conflict: in the
coördination, turn round, as it were, upon the seeing, and
hold it from passing over into further act until its quality is
determined. Just here the act as objective stimulus becomes transit
formed into sensation as possible, as conscious, stimulus. Just
ere also, motion as conscious response emerges.

In other words, sensation as stimulus does not mean any particular
psychical existence. It means simply a function, and will
have its value shift according to the special work requiring to
be done. At one moment the various activities of reaching and
withdrawing will be the sensation, because they are that phase
of activity which sets the problem, or creates the demand or,
the next act. At the next moment the previous act of seeing will
furnish the sensation, being, in turn, that phase of activity
which sets the pace upon which depends further action. Generalized,
sensation as stimulus, is always that phase of activity requiring
to be defined in order that a coördination may be completed.
What the sensation will be in particular at a given time, therefore,
will depend entirely upon the way in which an activity is being
used. It has no fixed quality of its own. The search for the stimulus
is the search for exact conditions of action; that is, for the
state of things which decides how a beginning coördination
should be completed.

Similarly, motion, as response, has only a functional value. It
is whatever will serve to complete the disintegrating coördination.
Just as the discovery of the sensation marks the establishing
of the problem, so the constitution of the response marks the
solution of this problem. At one time, fixing attention, holding
the eye fixed, upon the seeing and thus bringing out a certain
quale of light is the response, because that is the particular
act called for just then; at another time, the movement of the
arm away from the light is the response. There is nothing in itself
which may be labelled response. That one certain set of sensory
quales should be marked off by themselves as 'motion' and put
in antithesis to such sensory quales as those of color, sound
and contact, as legitimate claimants to the title of sensation,
is wholly inexplicable unless we keep the difference of function
in view. It is the eye and ear sensations which fix for us the
problem; which report to us the conditions which have to be met
if the coördination is to be successfully completed; and
just the moment we need to know about our movements to get an
adequate report, just that moment, motion miraculously (from the
ordinary standpoint) ceases to be motion and become 'muscular
sensation.' On the other hand, take the change in values of experience,
the transformation of sensory quales. Whether this change will
or will not be interpreted as movement, whether or not any consciousness
of movement will arise, will depend upon whether this change is
satisfactory, whether or not it is regarded as a harmonious development
of a coördination, or whether the change is regarded as simply
a means in solving a problem, an instrument in reaching a more
satisfactory coördination. So long as our experience runs
smoothly we are no more conscious of motion as motion than we
are of this or that color or sound by itself.

To sum up: the distinction of sensation and movement as stimulus
and response respectively is not a distinction which can be regarded
as descriptive of anything which holds of psychical events or
existences as such. The only events to which the terms stimulus
and response can be descriptively applied are to minor acts serving
by their respective positions to the maintenance of some organized
coördination. The conscious stimulus or sensation, and the
conscious response or motion, have a special genesis or motivation,
and a special end or function. The reflex arc theory, by neglecting,
by abstracting from, this genesis and this function gives us one
disjointed part of a process as if it were the whole. It gives
us literally an arc, instead of the circuit; and not giving us
the circuit of which it is an arc, does not enable us to place,
to center, the arc. This arc, again, falls apart into two separate
existences having to be either mechanically or externally adjusted
to each other.

The circle is a coördination, some of whose members have
come into conflict with each other. It is the temporary disintegration
and need of reconstitution which occasions, which affords the
genesis of the conscious distinction into sensory stimulus on
one side and motor response on the other. The stimulus is that
phase of the forming coördination which represents the conditions
which have to be met in bringing it to a successful issue; the
response is that phase of one and the same forming coördination
which gives the key to meeting these conditions, which serves
as instrument in effecting the successful coördination. They
are therefore strictly correlative and contemporaneous. The stimulus
is something to be discovered; to be made out; the activity affords
its own adequate stimulation, there is no stimulus save in the
objective sense already referred to. As soon as it is adequately
determined, then and then only is the response also complete.
To attain either, means that the coördination has completed
itself. Moreover, it is the motor response which assists in discovering
and constituting the stimulus. It is the holding of the movement
at a certain stages which creates the sensation, which throws
it into relief.

It is the coördination which unifies that which the reflex
arc concept gives us only in disjointed fragments. It is the circuit
within which fall distinctions of stimulus and response as functional
phases of its own mediation or completion. The point of this story
is in its application; but the application of it to the question
of the nature of psychical evolution, to the distinction between
sensational and rational consciousness, and the nature of judgment
must be deferred to a more favorable opportunity.